AdventOfCodeDay4.cs

using System; 
using System.Security.Cryptography;
using System.Text; 

namespace AdventOfCodeDay4
{
	class Program
	{
		static void Main()
		{
			var input = "ckczppom";

			using (var md5Hash = MD5.Create())
			{
				var found5 = false; 

				for(var i = 0; ; i++)
				{ 
					var hash = GetMd5Hash(md5Hash, $"{input}{i}");
					if (!found5 && hash.StartsWith("00000"))
					{
						Console.WriteLine(i);
						found5 = true; 
					}
					if (hash.StartsWith("000000"))
					{
						Console.WriteLine(i);
						break;
					}
				} 

			}

			Console.ReadLine();
		}

		static string GetMd5Hash(MD5 md5Hash, string input)
		{
			var data = md5Hash.ComputeHash(Encoding.UTF8.GetBytes(input));
			var sBuilder = new StringBuilder();
			foreach (byte t in data)
			{
				sBuilder.Append(t.ToString("x2"));
			}
			return sBuilder.ToString();
		}
	}
}